Filters for heating, ventilation and air conditioning systems (HVAC) play an important role by preventing cooling coils and heating surfaces from becoming coated with dust particles, which would result in a loss of efficiency and waste of energy. After a period of operation, dust accumulates on the filters, causing them to become clogged and requiring additional energy consumption. At some point it becomes cost-effective to remove the filter and either clean it or replace it with a new one. HVAC equipment manufacturers typically state their warranties so that the user, not the manufacturer, is responsible for equipment failures due to neglect in maintaining the system, with emphases on the system dust filter. A need exists for a device capable of monitoring the extent of accumulated dust on such filters and for providing a perceptible indication when obscuration of the filter reaches a predetermined level correlated with an optimum replacement time.
Various clogged air filter detection devices based on differential pressure monitoring have been developed in prior art for use in higher air velocity commercial systems, but devices of this type are ineffective for low velocity residential and commercial HVAC systems. Optical devices based on measurement of light passed through or reflected from air filters are also disclosed in certain prior patents. U.S. Pat. No. 5,141,309 discloses an optical system for detecting clogging of a specific type of vacuum cleaner dust filter which employs a pleated fabric filter. This patent shows a forked light unit having one arm carrying a light emitter and the other carrying a receiver so that a pleat of the filter may be placed between the arms in position for the light to be passed through the filter twice. In this device both arms are located on the same side of the filter so that access to the opposite side of the filter is not required. This patent also disclosed a forked light device with one arm disposed on one side of a flat filter and the other arm on the opposite side so that light passes through the filter and another embodiment wherein both the emitter and the receiver are located on the same side of a flat filter and light is directed against the filter surface to obtain a measure of filter loading with dust based on changes in the intensity of the reflected beam. U.S. Pat. No. 3,985,528 discloses an optical system incorporated as a control means for an “automatic roll-type filter” assembly, with a “photocell” indication when to advance clean filter material across the air duct. Placement of arms of a support element on opposite sides of the filter material for passing of light through the material is also disclosed.
The patents discussed above fail to recognize or provide a solution to certain problems presented in developing practical detectors using optical components for HVAC systems. Many of the filters deployed in these systems are located in a position such that very little space is available for mounting of necessary optical components. Filters are commonly placed adjacent to return air grills either touching or almost touching the grills so that no space is available for supporting a transmitter or receiver at any significant distance away from the filter on one side. In addition the nature of such filters imposes requirements for flexibility in placement of the transmitter and receiver. Both faces of the filter may be covered by a thin framework of containment sheets of metal foil provided with large circular openings or a cardboard lattice arrangement allowing air flow but restraining the filter material in place. In order for the transmitter, receiver and reflector to function properly they must be capable of being aligned with one another so as to provide an unobstructed path for passage of a beam of light through the openings.
An additional requirement arises from a need to place the transmitter and receiver at an offset angle with respect to one another so as to avoid partial blocking of flow through the filter at the point being monitored.
Peripheral framework for these filters typically comprises a U-shaped border made of flexible cardboard strips which would not provide enough rigidity to allow component support arms to be supported by this framework in a fixed position, as is required once the components are placed in proper alignment. A more stable base is therefore needed for securing of supports to which monitoring components are attached.
In addition to providing for the effective determination of a level of clogging in filters, detector systems including other features and characteristics would be desirable. In particular, such features would include use of miniaturized electronic components to compensate for the limited availability of space and providing programmable chips for performing such functions as calibrating the system and preparing a schedule for monitoring services.